The principal dimensions are as follows:—
| Feet. | In. | |
| Depth of the screw-blades below the sand, about | 14 | 6 |
| Depth of the screw-blades below low-water mark spring-tides | 21 | 0 |
| Rise of spring-tides | 15 | 0 |
| Height from high-water mark spring-tides to floor of house | 20 | 6 |
| Height from high-water mark to floor of light-room | 29 | 6 |
| Height from high-water mark to lamp | 38 | 6 |
| Height from high-water mark to top of vane spindle | 54 | 0 |
| Diameter of floor of house | 27 | 0 |
| Diameter of platform | 21 | 0 |
| Diameter of light-room | 12 | 0 |
A lighthouse of this kind is excellently adapted for any locality where the light does not require to be seen at a great distance. The piles offer no appreciable opposition to the waves, which pass through the open spaces without rising higher than out at sea.
The Gunfleet Lighthouse stands on seven screw piles, screwed 40 feet deep into the sand. The Point of Ayre Lighthouse, on nine, screwed 12 feet into the sand.
Before we conclude these desultory notes, it seems desirable to refer to a lighthouse now in course of erection, which is not unworthy to rank with the finest of its predecessors.
About midway between the famous Skerryvore Lighthouse and that of the Rhins of Islay—or 20 miles from Islay, 18 miles from Colonsay, 15 miles from Iona, and 15 miles from Mull—in the centre of an archipelago which ancient legend, and ecclesiastical history, and modern romance have done their best to render celebrated—lies the Dubhe Artach (or St. John’s) Rock. It forms an isolated mass of augite about 240 feet in length by 43 feet in breadth, whose rounded summit rises 47 feet above high-water mark. In stormy weather the sea sweeps over it with terrific violence, and for miles around it boils and seethes with counter-currents and opposing waves. During the severe gales of the winter of 1865–66 many ships were lost in this dangerous neighbourhood, and it was therefore determined by the Commissioners of Northern Lighthouses, with the sanction of the Trinity House and Board of Trade, to erect a lighthouse on the Dubhe Artach.
The material of the rock is so excessively hard that the works, at first, could not be carried forward with much rapidity. Neither in the building of the Eddystone nor of the Skerryvore could the engineers have had greater difficulties to contend with. A foundation has, however, been at last obtained, and several courses of the masonry securely laid, so that the elegant structure, designed by Messrs. D. and T. Stevenson of Edinburgh, will, in another twelvemonth, be completed. Its estimated cost is £56,900. It consists of a parabolic frustrum, whose topmost course is 109 feet above its base. The diameter at the bottom measures 36 feet, at the top 16 feet. There will be seven apartments besides the light-room. The total height of the lantern above the sea will be 154 feet, commanding a range of about eighteen miles.